Is it possible to possess an Actor through a widget?

Posses node is and will be always server only, only server can manage players.
So you need to tell server that you want posses some pawn as player.
Split whole task into separate parts:

  • tell player controller, you pressed widget to posses
  • player controller tells server side player controller that you want to posses something
  • server unposses old pawn and posseses new one.